spitfire 1500 rad cap

[Buns]

I am wanting to replace the radiator cap on a 1500 spitfire. I have been on rimmer brothers site and there are 2 caps. One is for an original radiator and one for a replacement. The original is a 7lb one and the replacement is a 13lb. Does anyone know why they would be different. Thanks

[//j17]

A 7lb cap was specified for the Spitfire Mk 1/2/3.
A 13lb cap was specified for the Spitfire Mk IV/1500.